FYI, I'm in SW 2016 sp 4 and do not have access to test other versions.

In a new Solidworks part, I create a 2D sketch on the front plane of the outline profile of a bottle. All the key dimensions of this shape are made referencing a single global variable (A). Then I create a second sketch on this same plane and use Convert Entities to convert this outline profile. In this sketch I use offset entities to add a wall thickness and add some tangent or fillet sketch lines to smoorth out parts of the design. I then use this second sketch to revolve and create the physical bottle. I create a third sketch and use convert entities to reference the second sketch's offset line, I fill out this sketch (so that it can revolve), and then revolve it. This third sketch revolved gives me the bottle's internal volume.

I now need to adjust the gobal variable (A) so that the bottle can scale up so that I can increase the internal volume to hit my target volume. My problem is, that when I change global variable (A), the first sketch updates fine, but the second one that references the first (through convert entities) does not update. The error on the second sketch reads:

Warning: The sketch has not been updated being solving it would result in invalid geometry (such as zero-lenght line). Consider:

As Ruben Balderrama indicated, having the file to look at would help (see How can I attach a file to a forum post?), but in any case that error message typically appears when for example an arc is too small to allow for the other dimensions so that lines would interfere or cross. That would be the first thing I'd check.

I had to replace the splines with several arcs to recreate the "S" curve.

I had to draw one arc, offset the neighboring straight line, then draw a new arc from the old arc's center point and connect it to the newly offset line (and make it tangent). Therfore the arc was not offset, but rather it connected to an offset line and would adjust accordingly.

While Rubén Rodolfo Balderrama's solution would be in an ideal world, I couldn't shell as I was dealing with uneven wall thicknesses. Therefore I had to draw both lines.